Flat Roof Drainage: Why Water Sits on Chicago Roofs

Flat roof drainage is the whole path water takes from the moment it lands on your roof to the moment it leaves your building. A Chicago flat roof is not meant to be flat. It is meant to have just enough slope to move water to a drain, a scupper or a gutter, and from there to a downspout. When water sits on the roof, one link in that chain is not working, and finding which one is the whole job.

This article covers the system. For the symptom of standing water and what it does to a roof, see ponding water on a Chicago flat roof.

How is a flat roof supposed to drain?

The Asphalt Roofing Manufacturers Association (ARMA) recommends at least 1/4 inch of slope per foot on a low-slope roof. It also quotes the National Roofing Contractors Association (NRCA) Roofing Manual as recommending positive slope through a sloped deck or framing, tapered insulation, and proper placement of drains, scuppers and gutters (ARMA).

That sentence names the three ways a roof gets its fall:

1. The structure is sloped. The deck or framing is pitched toward the drain from the start.
2. Tapered insulation. Insulation boards cut thicker at one end build slope on top of a level deck.
3. Drains placed at the low points. A drain in the wrong place does nothing for a roof whose low point is somewhere else.

Older Chicago flat roofs often rely on a combination. The slope that was there when the roof was built can change as layers are added, the deck settles or insulation compresses.

What do drains, scuppers and gutters actually do?

Three names, three ways out.

• Roof drain. An opening in the roof surface, usually fitted with a strainer, that carries water down through the building by a pipe.
• Scupper. An opening cut through the parapet wall or roof edge so water leaves sideways, usually into a leader or downspout.
• Gutter. A channel along the roof edge that collects water and sends it to a downspout.

Each has the same weakness: it only works if it is open. Leaves, seeds, grit and plastic caught against a strainer or in a scupper mouth will block it, and a blocked drain makes a pond.

Do flat roofs need an overflow drain?

Often. ARMA notes that secondary (overflow) drains may be required by local plumbing code (ARMA). We have not found a Chicago-specific rule we can quote, so check with the Department of Buildings (DOB) on your building before assuming either way.

The reason overflow matters: if the primary drain clogs during a heavy rain, water has to go somewhere. Without an overflow route, it rises on the roof until it finds a seam, a flashing or the weakest wall. That is why a roof that is fine in light rain can leak in a storm.

Why does water sit on a Chicago flat roof?

The drainage-side causes, in the order we usually check them:

1. A blocked drain, scupper or gutter. The easiest one to fix and the most common.
2. Drain not at the low point. The roof was re-sloped, resurfaced or settled, and the drain stayed where it was.
3. Not enough slope. The 1/4 inch per foot figure is a recommendation from industry guidance, not a Chicago code number (ARMA). A roof built dead flat has no margin.
4. No crickets or saddles. ARMA says crickets upslope of rooftop equipment and saddles along the low points between drains reduce localized ponding (ARMA). Our explainer on flat roof crickets covers what they are.
5. Added layers and patches. A repair that sits proud of the roof can dam water behind it, and extra layers can change the fall.

Where does the water go after it leaves the roof?

A lot of it, fast. The University of Minnesota Extension figures a 1 inch rain puts about 1,250 gallons on a 2,000 square foot roof (University of Minnesota Extension). A typical Chicago two-flat roof is not far from that size, so the downspout and what it discharges onto matter.

The same source says downspouts need extensions or splash blocks that discharge at least 4 feet from the wall, and that soil should slope away from the foundation (University of Minnesota Extension). That guidance is about basement water in general, not a Chicago code, but the logic is the same here: a roof that drains perfectly into a downspout that dumps against your foundation just moves the problem.

How can you check your own roof's drainage?

You can learn a lot without climbing up. Do the first items from the ground or a window.

• Look at the roof two days after a rain. Is water still standing? (ARMA's definition of ponding is 48 hours or longer.)
• Look for downspouts that are loose, disconnected or discharging against the building
• Look for stained or streaked parapet walls below a scupper
• Look for ceiling stains near the roof edge
• Only if it is safe and the roof is dry: check that every drain strainer and scupper is clear of debris

If you cannot get on the roof safely, do not. A roofer can inspect it, and the fall and spring flat roof checklist lists what to look at each season.

How is flat roof drainage fixed?

It depends on why the water sits. In rough order of how much work each is:

1. Clear the drains and scuppers. The cheapest fix and the first one to try.
2. Repair or replace a damaged drain or scupper, including the flashing around it.
3. Add crickets or saddles to direct water around equipment and toward drains.
4. Add tapered insulation so the roof gets its fall back during a re-roof. See what Chicago requires when you re-roof.
5. Add or relocate a drain. This one touches plumbing, which affects the permit.
6. Rebuild the slope. On a deck that is dead flat, the only real fix may be a structural one.

We are not quoting costs. Each rung is a different job, and what drives cost is the access, the number of drains, the condition of the deck and whether it is part of a bigger re-roof.

Does drainage work need a permit?

Often, and especially if you touch plumbing. The City says a repair is exempt from a permit only if it does not involve cutting away part of an exterior wall or roof, and does not involve work on mechanical, electrical, plumbing or fuel gas systems (City of Chicago). Moving or adding a roof drain is plumbing. Separately, the City requires a permit for work on low-slope roofs (City of Chicago), and reroofing must meet the energy code insulation minimums, which matters when tapered insulation is part of the fix (City of Chicago). Most reroofing permits go through the Express Permit Program, with limits on what Express covers (City of Chicago). Confirm your job with the Department of Buildings.

What to ask a roofer about drainage

1. Where is the low point of my roof, and where are the drains relative to it?
2. Is there an overflow route if the main drain clogs?
3. How will you fix the slope, if it needs fixing, and what does it do to the parapet and scupper heights?
4. Does your plan involve plumbing, and who pulls that permit?
5. Where do the downspouts discharge?

Roof drainage is a core part of every flat roof we look at, and a repair or re-roof that ignores it will not hold. For the full picture of the roof system, see the Chicago roofing guide, and when you want someone to look at yours, our roofing team will walk the roof with you.

Common questions

This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.